Trilobites were one of nature’s greatest success stories being found across the entire planet for almost a staggering 300 million years. One of the most recognisable trilobites is the Phacops, and when I was researching for this post I saw that this was just the tip of the iceberg. As a result, I wanted to expand on this post to talk about the family of trilobites called Phacopidae – as many species of Phacops have been reassigned to other members of the family, I thought it would be better to talk about the entire family. These tiny, woodlouse like invertebrates were incredibly successful, and while they may not be super impressive in looks, they are an incredible insight into the evolution of life.
Discovery and Fossils
The first fossils of Phacops was originally discovered in 1825, but were originally identified as belonging to a different genus of trilobite called Calymene. In 1839 German geologist Hermann Friedrich Emmrich realised that a few species of Calymene were actually a separate genus which he called Phacops. In my research I was unable to properly find out what Phacops means, but from the awkward sources which I found it seems that it roughly means ‘Lens face’ – a reference to the iconic and well-preserved eyes of the trilobite. Currently, there are 25 confirmed species of Phacops, with Phacops latifrons being the first to be named. However, a lot of trilobites closely related to Phacops over the years have been incorrectly, but understandably, classed as Phacops – most notably Eldredgeops rana. Named rana, (Rana being the genus which frogs belong to), as from the front it looks like a little frog’s face, Phacops rana became an incredibly popular fossil for collectors. In 1990 it was realised that Phacops rana actually belonged to a different genus called Eldredgeops, but the original classification has remained among collectors – my own Phacops, pictured above, most likely is an Eldredgeops.
Today we’re talking about the family Phacopidae, not the order Phacopida because these trilobites become to diverse to talk about in one post for my liking. I might do a post talking about trilobites as a whole, but that’s for another day. The fossils belonging to Phacopidae are extremely common with Phacops and Eldredgeops being the most common. Both of these are especially common in the northeast US, southern Ontario, and Morocco (where my trilobite hails from). This might seem strange, but when the trilobites were scuttling around the sea bed North Africa and North America were much closer together. As most Phacopidae trilobites scuttled along the sea floor this meant that many were covered in silt and preserved very well in large numbers. As a result, we have countless Phacopidae fossils, especially Phacops, Reedops, and Eldredgeops, and you can buy them from any fossil collector fairly regularly.
When and Where
Where is very simple: everywhere. Phacopidae have been found on every continent in most countries. This is not simply limited to the family as a whole, some genus are found globally – most notably Phacops. Here are a few examples: the blind Ductina is found across Europe, Vietnam, and south-east China; Toxophacops is from Japan; Adastocephalum from Australia; Afrops from southern Algeria; Reedops from Argentina and Africa; and you can just imagine how common this family was. The Phacopidae existed on this planet for a staggering 100 million years! If we traced our family tree back 100 million years, our ancestor would be a opossum-like marsupial under the feet of dinosaurs. First appearing in the Late Ordovician around 450 million years they were found until the end of the Devonian, around 358 million years ago. Phacops actually existed as a genus throughout this time. The Phacopidae even managed to survive a mass extinction event, the Ordovician-Silurian extinction event, which wiped out 50% of all trilobite families. It took another extinction event to wipe of the Phacopidae.
Biology
First, we need to have a look at the biology of trilobites as a whole. Trilobites got their name because their bodies are divided into three: a central axial lobe, a left pleural lobe, and right pleural lobe. You can then divide trilobites from head to tail: the cephalon (which contains the eyes, mouth, and a lobe called the glabella where the digestive system is), the thorax, and the pygidium. The image above, albeit from Wikipedia, shows this biology well. While trilobites had different features, they had this body plan. Under the pleural lobes there were the gills and legs, and many species would have antennae which rarely (if ever) fossilised. Trilobites fossilised well as their exoskeletons were made of chitin, a carbohydrate which forms the exoskeletons of modern arthropods like crustaceans and insects. Trilobites are also known for their beautifully preserved eyes, although not all trilobites had eyes. The Phacopidae were generally small trilobites ranging from the size of your thumb to being able to fit in the palm of your hand. The Phacops I have, pictured below, shows how this.
The Phacopidae had some of the best preserved eyes of all invertebrates, and had some of the best vision of its time. That is, the Phacopidae which had eyes – Ductina actually lacked eyes. Trilobites had a unique set of eyes which are called schizochroal eyes, which are especially prominent in the Phacopidae trilobites. Their eyes were made of individual lenses arranged in hexagonal patterns separated by walls made of cuticles. The eyes are so well preserved that scientists in the 1970s managed to analyse the composition of the eyes revealing that the lenses were made of calcite crystals making the eyes like glass. The lenses were further made of two components which could refract light. This gave Phacopidae trilobites incredibly sophisticated vision which could be sharply focused, and allowed them to look over a wide area. Not only would this allow trilobites like Phacops to detect predators, but also hunt prey themselves.
Another key part of the Phacopidae biology is their ability to roll into a ball like modern woodlice and armadillos. It is not uncommon to find these trilobites preserved in curled up balls. While the exoskeleton of the trilobite was made of tough chitin, the underside was not, and this left the legs, gills, and internal organs open to attack. By curling up this helped secure the soft underbelly of the trilobite. Phacops, Reedops, and Eldredgeops were also noted for having evolved the ability to efficiently curl into a ball. Using Phacops as an example, the thorax was divided into 11 segments in order to make it easier to curl up, had a small tailshield which could be tucked under the head, and had smooth sides to its pleural lobes to better form a ball.
Life of the Trilobites
As trilobites are generally well preserved this has given us a good insight into the lifestyle of trilobites. The Phacopidae were mostly active arthropods making their home in the corals and seabeds in shallow seas. Several species have been found with darker spots on their exoskeletons since the 1960s, which has led to the theory that Phacopidae trilobites had these dark spots in order to camouflage with the sea bed. This would add a further protection against potential predators, but also as a way to get close to prey items. The actual mouth parts of trilobites, due to them being soft, have not been fossilised, so we cannot fully know what they would have eaten. It is clear, however, that Phacopidae would largely be detritovores – meaning that they would eat the detritus on the sea bed. The blind Ductina likely would have been a detritovore, possibly even hiding in the sand to pick up bits of plankton, smaller organisms, and potential decaying organisms. Meanwhile, the brilliant eyesight of Phacops, for example, has led palaeontologists to suggest that they might be more active hunters. We wouldn’t see trilobites hunting down fish or other animals, but instead would pick up smaller organisms and plankton. For example, smaller trilobites, small fish, eggs, worms, and plankton could all be on the menu. I bring up smaller trilobites as arthropods shed their exoskeletons in order to grow larger. While the new exoskeleton is growing this leaves arthropods lacking their main defense, so a smaller trilobite could be eaten up while molting.
Extinction
The Phacopidae trilobites were an incredibly successful group, and had already survived a mass extinction. Two major factors led to the extinction of this group of trilobites. The first, jawed fish had evolved during the Devonian. Being able to bite down with some force made trilobite armour a lot less effective against deterring predators, so trilobite populations dropped. Not only Phacopidae trilobites were affected, but all families of trilobites. You can see this in fossil evidence, as trilobites began evolving spikes. However, it would be another mass extinction which would wipe out the Phacopidae trilobites. The causes of the Devonian extinction is still not fully understood, but several factors could be behind it. A mixture of climate change and volcanic activity has been posited as a possible cause, as the two would be large enough to drastically change environments which many organisms relied on. Around 50% of all genra were wiped out, and most of the trilobites – including all of the Phacopidae – went extinct. However, while the Phacopidae trilobites went extinct, not all trilobites did. It would take the worst extinction event to ever hit the planet to drive trilobites into extinction.
Bibliography:
- Angeles Gavira Guerrero and Peter Frances, (eds.), Prehistoric Life, (London: 2009)
- Ben G Thomas, ‘Triumph of the Trilobites’, YouTube.com, (17/02/2019), [Accessed 20/03/2022]
- PBS Eons, ‘The Trouble with Trilobites’, YouTube.com, (26/06/2017), [Accessed 20/03/2022]
- Riccardo Levi-Setti, The Trilobite Book, (Chicago: 2014)
- Jonathan Adrian and Eugene MacDonald, ‘Phacopid Trilobites from the Silurian of Arctic Canada’, Journal of Paleontology, 70:6, (1996), 1091-1094
- David Holloway and Juan Rustan, ‘The Trilobite Reedops (Phacopidae) in the Lower Devonian of Argentina (Malvinokaffric Realm)’, Journal of Paleontology, 86:2, (2012), 253-257
- David Fordyce and Thomas Cronin, ‘Trilobite Vision: A Comparison of Schizochroal and Holochroal Eyes with the Compound Eyes of Modern Arthropods’, Paleobiology, 19:3, (1993), 288-303
- Paul Zell, ‘Burrowed Phacops rana from the Moscow Formation of New York’, Journal of Paleontology, 62:2, (1988), 311-312
- Ivo Chlupac, ‘The distribution of phacopid trilobites in space and time’, Fossils and Strata, 4, (1973), 399-408
- Christopher Burton and Niles Eldredge, ‘Two new subspecies of Phacops rana [Trilobita] from the Middle Devonian of North-West Africa’, Palaeontology, 17:2, (1974), 349-363
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